A new way to find distance of gamma-ray burstsRICE UNIVERSITY NEWS RELEASEPosted: January 12, 2002

Astronomers at Rice University in Houston have discovered that the rate
at which a gamma-ray burst cools might be used to calculate the distance
of that burst.

Their findings are being presented this week at the American Astronomical
Society meeting in Washington, D.C. The researchers believe that this
additional technique will enable scientists to learn more about the
evolution of the early universe.

Gamma-ray bursts are transient, short flashes of gamma rays that occur
randomly in the sky every day. The gamma rays themselves cannot be seen
by human eyes, but astronomers' instruments in orbit around Earth can
detect them. Since 1997, scientists have known that these bursts
represent gigantic explosions likely associated with the death of
massive stars at a distance of about 10 billion light years in the
early part of the universe. By determining the distance of gamma-ray
bursts, astronomers hope to trace the formation of massive stars and
the structure and evolution of the early universe.

About 3,000 gamma-ray bursts have been recorded, mostly during the
1990s, but astronomers know the actual distance to only a very few
bursts. In recent years, two methods have been proposed for indirectly
calculating the distance from the available data. Edison Liang and Dan
Kocevski at Rice University, collaborating with Brad Schaefer at The
University of Texas at Austin, have come up with a third.

"It's well-known that gamma-ray bursts start at high energy and evolve
to lower energies," said Liang, a professor of physics and astronomy
at Rice. Gamma-ray spectrometers convey this shift in energy through
changes in color, going from blue (gamma rays with high energy) to red
(lower energy).

"We examined 16 gamma-ray bursts and found that the apparent rate at
which the burst is cooling off appears to be directly related to the
distance of the burst, provided that the rate is measured not in terms
of time, but in terms of the total number of gamma rays emitted since
the beginning of the pulse," Liang said.

But this technique works only on gamma-ray bursts that have separable
pulses, or peaks, of intensity. Bursts that are "chaotic" have multiple
peaks, or spikes, of energy. The combination of data from multiple
overlapping gamma-ray pulses makes it difficult to estimate the true
cooling rate of the highest peak, according to Kocevski, a graduate
student at Rice. "You tend to underestimate the cooling rate when
observing bursts with multiple peaks," he said.

Since the majority of gamma-ray bursts are of the chaotic variety,
Liang and Kocevski are now trying to develop methods to separate
the color of overlapping pulses from within the chaotic bursts to
determine the true cooling rate. Using software the Rice group
developed to measure the cooling of bursts, the researchers are
hopeful that they will be able to apply their technique for
calculating a gamma-ray burst's distance to chaotic bursts. This
would expand the database of knowledge from which deductions about
the formation of the early universe can be made and provide new
insights into the physical mechanisms of these enigmatic explosions.

"It's very labor-intensive and tedious, but we have high hopes it will
work eventually," Liang said.

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